Pultruded profiles are well-known in the art and are generally formed by continuously pulling a reinforcing material through a curable or polymerizable liquid resin matrix and then through a die having a desired cross-section to shape the impregnated reinforcement material into a cured product having continuous length and a uniform cross-section that was formed by the die. Other methods such as injecting thermoset or thermoplastic resins into the reinforcing material or using pre-impregnated reinforcing material will yield similar results. Part cure can be achieved by heating the die with electric cartridge heaters or hot oil pumped through channels in the die. Also radio frequency, hot air heating or ultra violet radiation techniques have been used to effect a partial or full cure to resins for part solidification. These processes result in strong, yet light weight, parts that can be cut to any desired length.
Unfortunately, the surface characteristics of these pultruded profiles have some disadvantages. First, the reinforcing fibers are very close to the surface of the part, say less than 0.0005 inches. Therefore, any small amount of surface degradation (which can be a color change and/or erosion of the resin) from ultraviolet exposure, abrasion, chemical attack, wind erosion or the like will expose the reinforcing fibers. Exposing the reinforcing fibers to the elements will create an unsightly looking part and/or a part that is uncomfortable to handle. Continued exposure will cause both the resin matrix and reinforcing fibers to be eroded and eventually the physical properties of the part will be adversely effected, reducing the parts effective useful life.
One method for solving this problem is to use high performance resins instead of the commonly used pultrusion resins. Unfortunately, this method results in a substantial cost increase in the parts because 30% to 70% of the finished part is made up of the resin matrix.
A second method for solving this problem is to use a surfacing mat such as a polyester veil. This method will result in some increase in the part cost but more importantly it does not protect the part from surface degradation such as discoloration and chalking. Therefore, it is not an acceptable solution for many applications where appearance over extended periods of time, say greater than 5 years, is important. However, it does significantly increase the length of time before the reinforcing fibers become exposed to the elements. A third method involves the addition of additives such as UV-5411, which is manufactured by the American Cyanamid Company, to the resin matrix. This can slow down the erosion process but only provides marginal improvement and again is not an acceptable solution in many applications. It is common to combine methods two and three for best results with this type of technology.
A fourth method for solving the surface erosion problems associated with pultruded profiles is to apply a 0.0005 to 0.0050 inch thick high performance coating material onto the part's surface. Applying a high performance coating, sometimes combined with previous techniques is the ultimate answer to protecting the surface of a pultruded part. Normally, this is done as a separate operation using either powder or high solvent coatings that are sprayed onto the part and then cured by passing the parts through ovens. In both cases, final part costs increase substantially due to the extra labor for part handling, high equipment costs, high energy costs, and a significant increase in the amount of total space required. Also, the elapsed time to produce a finished part is increased substantially. In addition, spraying always involves a certain amount of waste due to over spray which creates both cost and disposal problems. Over spray is even a greater problem with most pultruded parts because they typically are not conductive, therefore electrostatic spray methods, a common method of reducing over spray, cannot easily be used. Finally, liquid spray coatings contain a high level of solvents. These solvents have to be evaporated off which in turn create environmental problems.
Therefore, there arises a need for a cost-effective method for applying a coating material to the surface of pultrusion profiles during the pultrusion process that are considered environmentally sound.